1. Field of the Invention
The present invention relates to a magnetic sensor system including a magnetic field generation unit for generating a magnetic field to be detected and a magnetic sensor for detecting the magnetic field.
2. Description of the Related Art
In recent years, magnetic sensors have been widely used to detect the rotational position of an object in a variety of applications such as detection of the degree of opening of a throttle valve in automobiles, detection of the rotational position of steering in automobiles, and detection of the rotational position of the wiper of automobiles. Magnetic sensors are used not only to detect the rotational position of an object but also to detect a linear displacement of an object. Systems using magnetic sensors are typically provided with means (for example, a magnet) for generating a magnetic field to be detected whose direction rotates in conjunction with the rotation or linear movement of an object. Hereinafter, the magnetic field to be detected will be referred to as the target magnetic field. The magnetic sensors use magnetic detection elements to detect the angle that the direction of the target magnetic field in a reference position forms with respect to a reference direction. The rotational position or linear displacement of an object is thus detected.
Among known magnetic sensors is one that employs a spin-valve magnetoresistive (MR) element as the magnetic detection element, as disclosed in WO 00/17666, U.S. Pat. No. 7,483,295 B2, U.S. Pat. No. 7,394,248 B1, and U.S. Pat. No. 8,054,067 B2. The spin-valve MR element has a magnetization pinned layer whose magnetization direction is pinned, a free layer whose magnetization direction varies according to the direction of the target magnetic field, and a nonmagnetic layer disposed between the magnetization pinned layer and the free layer.
A magnetic sensor that employs a spin-valve MR element as the magnetic detection element may have an error in a detected angle due to variations in the magnetic properties of the MR element, as described in U.S. Pat. No. 8,054,067 B2. U.S. Pat. No. 8,054,067 B2 discloses a technology for reducing an error in the detected angle caused by manufacturing variations in MR elements. This technology is, so to speak, a technology for reducing an error in the detected angle that will be found at the time of completion of the magnetic sensor as a product.
Errors in the detected angle that could occur in the magnetic sensor include an error that emerges after the installation of the magnetic sensor in addition to an error found at the time of completion of the product as mentioned above. One of the causes by which an error in the detected angle emerges after the installation of the magnetic sensor is an induced magnetic anisotropy that occurs on an a posteriori basis in the free layer of the MR element. Such an induced magnetic anisotropy may occur in the free layer when, for example, the temperature of the MR element is lowered from a high temperature while an external magnetic field is being applied to the MR element in a particular direction. Such a situation may occur when, for example, the magnetic sensor is installed in an automobile and a specific positional relationship is established between the magnetic sensor and means for generating a target magnetic field during non-operation of the automobile. More specifically, the aforementioned situation may occur when the magnetic sensor is used to detect the position of an object that comes to a standstill in a predetermined position during non-operation of the automobile, such as the wiper of an automobile.
The magnetic sensor is required to have a reduced error in the detected angle that may emerge due to an induced magnetic anisotropy occurring on an a posteriori basis after the installation. Note that the foregoing descriptions have dealt with the problem that is encountered when an induced magnetic anisotropy occurs on an a posteriori basis in the free layer of a spin-valve MR element after the installation of a magnetic sensor that employs the spin-valve MR element as the magnetic detection element. However, this problem applies to any cases where the magnetic sensor has a magnetic detection element that includes a magnetic layer whose magnetization direction varies according to the direction of the target magnetic field and an induced magnetic anisotropy occurs on an a posteriori basis in the magnetic layer of the magnetic detection element after the installation of the magnetic sensor.